How does the nature of the solute affect boiling point?

The solute raises the boiling point by an amount that depends on the number of particles it contributes go the solution.

Boiling point elevation is a colligative property. It depends only on the number of particles in the solution. The formula for boiling point elevation is

#ΔT_b = iK_b m#

where m is the molality of the solution, #K_b# is the molal boiling point elevation constant for the solvent, and i is a number related to the number of particles the solute contributes to the solution (the van’t Hoff i factor).

Non-electrolytes do not dissociate when they dissolve. Thus, one mole of glucose will provide one mole of particles, and i = 1.

#ΔT_b = K_b m#

NaCl dissociates into Na⁺ and Cl⁻ in water. So if you have 1 mol of NaCl, you have 2 mol of particles and i= 2. For CaCl₂, i = 3, for FeCl₃, i = 4, etc.

#ΔT_b = iK_b m#

EXAMPLE

Calculate the boiling point of a 0.15 m aqueous solution of sodium chloride. #K_b# for water is 0.512 °C•m⁻¹.

Solution

#ΔT_b = iK_b m# = 2 × 0.512 °C·m⁻¹ × 0.15 m = 0.15 °C

#T_b# = 100.00 °C + 0.15 °C = 100.15 °C

Here is a video of a lab examining this topic.

Here is a video which shows some sample calculations of boiling point elevation.

videos from: Noel Pauller